Baked normal map from high poly bad corners
Let's start with vertext normals of smooth and solid shaded mesh:



Now see this diagram how baking works and projects the high-poly to lowpoly:



In each corner of your domino piece you see 3 corners baked, because of Repeated details. You can fix this by making the lowpoly domino shade smooth - that way there will be only 1 vertex normal in each corner, so you won't get overlaps and multiple projections:



If you want your domino silhouette to look better, you will have to add mode polygons. See: How can a Normal Map affect the look of a model?

There is also very comprehensive wiki about normals which might help: Polycount Normal map (the pictures are from there).